Entitlement subsystem

Overview and History

Kill Bill has been designed from day one with an entitlement system. The idea behind it, is to separate the billing view and the entitlement view (i.e the service given to the user) attached to a subscription. There are many situations where the two views don’t quite coincide: For example a user may be given access to a service (entitlement) prior being billed, or a user could cancel his subscription and yet the system might be configured to keep the billing running until the service was paid (EOT) to avoid pro-ration credit.

Kill Bill can therefore be used to decide which user has access to a specific service, and at the same time (independently) compute the correct amount a given user should be charged.

Implementation: Blocking State

The implementation of the entitlement module in Kill Bill is extremely powerful: It relies on the blocking_states table to insert rows that will control the entitlement associated with a subscription.

Each row inserted has the following attributes:

blockable_id : The id of the object being blocked/unblocked (see type below)

Time T2 : SVC2 unblock S ⇒ EntitlementState associated to S would still be BLOCKED

Time T3 : SVC1 unblock S ⇒ EntitlementState associated to S would now be ACTIVE

In addition, when services insert rows they need to specify a state. The state is opaque for the system but can be interpreted by the service itself (we will see examples below).

Note: It is mandatory to use different state name for a given service when blocking or unblocking a subscription otherwise the system will assume this is a duplicate and will prevent inserting the row.

Type Aggregation

The system allows inserting rows across different types: ACCOUNT, SUBSCRIPTION_BUNDLE, SUBSCRIPTION. As expected, inserting a blocking row for an ACCOUNT type would block all entitlement attached to that account.

For example, if a given service SVC insert the following rows for the subscriptions S1 and S2, all attached to the same account A:

Time T0 : SVC block A ⇒ EntitlementState associated to S1 and S2 would be BLOCKED

Time T1 : SVC block S1 ⇒ EntitlementState associated to S1 and S2 would be BLOCKED

Time T2 : SVC unblock A ⇒ EntitlementState associated to S1 is still BLOCKED but S2 would now become ACTIVE

Extensions

Originally, the blocking_states table was meant for services to insert rows that the service itself would know how to interpret (using the state) and that the system would know how to interpret by aggregating the block_entitlement flags across types and services to control the state of the entitlement.

The table now also includes a block_billing flag which also allows to block/unblock the billing associated to a specific type (ACCOUNT, SUBSCRIPTION_BUNDLE, SUBSCRIPTION). This mechanism can be seen as an overlay on top of the raw billing events to pause/resume billing. The invoicing system knows how to aggregate the raw billing events along with the blocking_states rows.

In effect the blocking_states table now offers functions wider than the pure entitlement system (and could be seen by purist as a design violation).

Use Cases

Kill Bill Modules

Two of the Kill Bill system modules currently rely on that mechanism to control the entitlement state associated with a subscription:

The entitlement module itself uses the service name entitlement-service to insert its own rows. For example pausing or resuming a bundle translates into inserting the correct rows into the blocking_states table. The entitlement module would also insert such a row when the user decides to cancel a subscription to make sure the state becomes CANCELLED.

The overdue module will also insert rows matching the overdue.xml configuration when the subscription transitions through states. The overdue module knows how to interpret the state associated with each row and take appropriate actions. In addition it can leverage the block_entitlement flag to control the entitlement state.

Plugins and Third Party Services

Plugins (or third party services) can leverage the blocking_states mechanism through apis to control the entitlement (and also billing through the use the block_billing flag) associated with each subscription.

Examples of such use case would be seen during a migration phase to control whether entitlement is active and when billing starts.